1. Field of the Invention
The present invention generally relates to concentration control techniques and in particular to method and apparatus for controlling a concentration of a solute in a solution such as a liquid developer by means of optical detection.
2. Description of the Related Art
In an electrostatic recording apparatus using a liquid developer composed of toner and solvent, an electrostatic latent image formed on an image carrier is developed by the liquid developer being in contact with the image carrier. Therefore, it is very important to keep the toner concentration of the liquid developer constant. In general, there has been used a method that detects the toner concentration of the liquid developer and then adjusts it by adding toner to the liquid developer so as to keep the toner concentration constant.
There has been proposed an optically toner concentration detecting technique making use of transmittance of liquid developer in Japanese Patent Unexamined Publication No. 62-124567. More specifically, a transparent pipe through which liquid developer flows is placed between a light source and a photodetector. Based on the output of the photo detector, transmittance of the liquid developer is detected and is used for toner concentration control.
However, there occurs an increase in amount of toner adhering to the inner surface of the transparent pipe with the passage of time and thereby the transparent pipe becomes a factor that substantially influences the toner concentration measurement of the liquid developer, resulting in a lower degree of measurement accuracy.
There has been also proposed another optically toner concentration detecting technique making use of electrophoresis. A pair of electrodes is provided within the liquid developer reservoir and a predetermined voltage is applied thereto. This causes toner particles to move and adhere to one of the electrodes due to the electrophoresis. By detecting the toner adhering to the electrode, the toner concentration of the liquid developer can be obtained.
However, such a toner concentration detecting apparatus making use of electrophoresis needs a power supply for supplying power to the electrodes, resulting in increased amount of hardware and thereby increased cost.